Wire rope slings made of a plurality of metal strands twisted together and secured by large metal sleeves or collars are common in the industry. During the past thirty years, industrial metal slings have seen improvements in flexibility and strength. However, compared to non-metal or synthetic fiber slings, metal slings are relatively stiff and inflexible.
Synthetic fiber slings have gained popularity over the last fifteen years and are replacing metal slings in many circumstances. Synthetic slings are usually comprised of a lifting core made of twisted strands of synthetic fiber and an outer cover that protects the core. The most popular design of synthetic slings is a roundsling in which the lifting core forms a continuous loop and the sling has a circular or oval-shaped appearance.
An advantage of synthetic slings is that they have a very high load-lifting performance strength-to-weight ratio which provides for a lighter, more flexible and even stronger slings than their heavier and bulkier metal counterparts. Even with such advances in the art of sling making, the riggers who use these improved synthetic slings still suffer and endure some of the age old problems of sudden failure and loss of a load caused by a sling breaking without warning because it was fatigued (or overly stretched) from being subjected previously to overload conditions. After a sling has been fatigued, it does not usually provide any physical indication that it was damaged—even to the trained eye. (One of the few advantages of a metal sling over a non-metal sling is that there is equipment available that can be used to conduct a non-destructive test of the metal. For example, similar equipment is routinely used to determine whether the wings of an airplane have become fatigued.)
Standard break tests have been established for determining how large of a load a sling can endure. Slings are attached to a testing machine that applies a steady but increasing force on the sling until it is unable to withstand the stress of the force being applied to it and the sling ultimately breaks. Such break tests have enabled manufacturers of industrial slings to rate the load-bearing capacity of the sling. The load capacity is determined to be a point well below the load used to break the sling and also below the point where the sling is fatigued or damaged. Most sling manufacturers will affix some type of tag notice on the sling which states the load capacity (rated capacity) of the particular sling. This rated capacity gives the maximum amount of load to which the sling may be subjected and still be considered a safe use of the sling.
Unfortunately, even conscientious operators/riggers who do not take unsafe shortcuts and who operate in a safe responsible manner sometimes are surprised by a sling breaking in use even when they believed it was being used within the load limits of its rated capacity. For example, when industrial slings are in continuous heavy use over three shifts around the clock, the operators on a later shift may not be aware that someone on an earlier shift had subjected the sling to a substantial overload which may have caused serious damage to the lifting core strands of the sling. When a synthetic fiber sling is overloaded beyond its tensile strength or weight-lifting capacity at maximum stretch, it is considered to be fatigued and may never return to its normal strength and load bearing capacity.
When subjected to an overload condition above its rated capacity, a roundsling can be permanently damaged/deformed if the load stretches the fibers of the load bearing core material beyond their yield point. An over-loaded sling may be susceptible to fracture at a stress point. This condition is similar to the stretching of a rubber band beyond its point of normal elasticity so that when the load or tension is removed or relieved, the rubber band will never regain its normal configuration and its strand dimensions may be permanently stretched which will cause it to fail under a load which is less than its tensile strength load. As stated previously, it is nearly impossible to determine, upon a cursory visual inspection, that a sling has been damaged because of the large size of such slings (on the order of 6 feet or more) and because the load-bearing core is hidden inside the outer cover.
Once the load-lifting core of the synthetic sling is stretched beyond its yield point, it can actually change in its physical structure and be restricted at a stress point. To date, there has been no precise method or apparatus available to an operator or rigger to determine if a sling with a protective cover was subjected to an overload or damage-causing condition. If a roundsling has been fatigued or structurally changed, the sling may no longer lift a load according to its maximum rated load capacity and, most importantly, becomes a serious threat to the operators and riggers using the sling.
Thousands of roundslings are being used on a daily basis in a broad variety of heavy load lifting applications which range from ordinary construction (e.g., skyscrapers and bridges), plant and equipment operations, to ship building (e.g., oil rigs), nuclear power plants and the like. The lifting core fibers of such roundslings may be derived from natural or synthetic materials, such as polyester, polyethylene, nylon, and the like. Although the outer covers of synthetic slings are designed to reduce damage, the core fibers are still susceptible to damage from abrasion, cutting by sharp edges, or degradation from exposure to heat, cold, ultraviolet rays, corrosive chemicals or gaseous materials, or other environmental pollutants.
In certain instances, the core yarn of a synthetic sling could weaken, melt or disintegrate when subjected to elevated temperatures, or to prolonged exposure to either ultraviolet light or chemicals. Still another safety concern flows from abuse by the user when the core yarn is damaged from abrasive wear when the slings are not rotated and the same wear points are permitted to stay in contact for extended periods of time with a device used for lifting (such as hooks on a crane), or on the edges of the load itself. Such abrasion is accelerated for certain types of synthetic fiber material and especially if the load contact section is under compression or is bunched. Riggers in the field are concerned that the inner lifting core yarn of their roundslings may be damaged on the inside without a means for them to detect such defects through the sling cover. Even if the cover is removed it may be impossible to tell if the lifting core has been damaged to the point where it cannot lift its rated load. Since there is no reasonable non-destructive testing techniques for synthetic fiber slings, a synthetic sling that is only suspected of being damaged must be removed from service for safety reasons.
The structural integrity of the roundsling lifting core material is difficult to determine when it is hidden inside a protective cover of opaque material which renders the lifting core yarn inaccessible for inspection. A stretched or fatigued roundsling could experience a sudden catastrophic failure without warning to the rigger, which may result in the loss of lives and property. Many in the industry have sought to provide safe slings to its riggers to avoid bodily injury, property damage and product liability claims.
Several roundsling constructions are known which have a failure indicator. For example, it is known in the art to incorporate a failure indicator synthetic strand as an integral member of the lifting or load-bearing core. The failure indicator strand in prior art constructions was always an extension of the core yarns.
A popular design of prior art roundslings was to twist a plurality of yarns together to form a single strand; the strand is then rolled into an endless parallel loops of strands that form the core, which is then encased in a protective cover material. If the sling was designed with a prior art failure indicator, an indicator strand would be incorporated into and twisted with the core yarns. The two ends of the indicator strand (sometimes referred to as tell-tails), extend freely through an opening in the cover material. When the sling is subjected to an overload condition, the tell-tail would partially withdraw within the cover and the freely extending tell-tail ends would be visibly shorter than the tell-tails of an undamaged sling; if the overload condition exceeded the maximum rated load of the sling, one or both tell-tails would usually withdraw completely within the cover. In either event, the rigger is warned of the occurrence of a potentially damaged sling by either the absence of one or both tell-tails, or a “significant” withdraw of at least one tell-tail inside the cover. However, there usually was no consistency on how the tell-tails would react when triggered, even when the slings were manufactured under identical conditions.
A drawback of prior art failure indicators based on an indicator strand is that there is no predictable way of determining when the failure indicator will be triggered. Synthetic slings have a safety factor designed into their construction. For example, if the sling is rated at 6,000 pounds, it typically will not be damaged unless the sling is subjected to a force five times greater (i.e., around 30,000 pounds, a 5-to-1 design factor) than the rated capacity; the tell-tail may be triggered and indicate an overload condition when the sling is subject to a force of between four to five times the rated capacity (i.e., about 24,000 lbs) by retracting into the sling's cover. Therefore, the tell-tail will provide a visual indication that the sling may have been damaged or subjected to a situation that may have been detrimental to the overall condition of the sling before the sling actually is subjected to such a condition. Unfortunately, there was no way of ensuring that the tell-tails would consistently withdraw within the cover at about 24,000 pounds.
In other words, two slings having prior art failure indicator strands contemporaneously made under the same conditions would have two different trigger points (for example, one sling may trigger at about 22,050 pounds and the other sling may trigger at about 26,000 pounds). In addition, one sling may react to a trigger event by completely withdrawing one of the tell-tails, while the other sling may react to a trigger event by partially withdrawing both tell-tails.
If the tell-tail is not withdrawn completely within the cover, one rigger's opinion of a “significant withdrawal” towards the opening in the cover may differ from another rigger's opinion. Therefore, a “small” movement of one or both of the tell-tails, which may result from the constant use and handling of the sling, may appear to one rigger as an indication that an overload condition was reached when, in fact, the sling was not subjected to an overload condition. Therefore, the visual inspection of the tell-tails in prior art failure indicators and the eventual determination of a trigger event becomes a subjective test.
Another prior art roundsling construction utilizes an optical fiber strand that enables the operator/rigger to test it by shining a light on one end of the optical fiber to determine if the light can be seen at the other end of the optical fiber. In U.S. Pat. No. 5,651,572 to Dennis St. Germain, it is taught to incorporate a flexible fiber optic “signal” cable into the lifting core strands of the roundsling.
As indicated previously, in a roundsling, the lifting core is configured in endless parallel loops of strands which are then encased within a protective cover material. The cover will have openings or orifice slits out of which the two ends of the fiber optic signal strand emerge. The aforesaid ends of the fiber optic cable are designed to extend freely through a slit in the sling's cover so that they are easily accessible by the rigger.
The optical signal strand member conducts light from a light source at one end to an observer looking at the opposite end for testing the integrity and the continuity of the core strands. The inclusion of the fiber optic cable in the lifting core yarn of the roundsling converts the inaccessible inner core area into an observable test check area by means of the passage of light through the fiber optic component of the lifting core.
Fiber optic materials are capable of transmitting light into endless parallel relationship with the fibers of the lifting core yarn. This fiber optic signal strand comprises fiber or rod material which permits the propagation of light that enters the fiber material at one end and is totally reflected back inward repeatedly from the fiber wall through the entire length of the fiber optic strand which enables the light being transmitted within the fiber optic cable to pass from one end of the fiber optic cable to the other end. If the light emerges at the other end of the fiber optic cable, it indicates that the integrity of the fiber optic cable throughout the path of the roundsling lifting core bundle is intact and, by reasoning, the integrity of the lifting core yarns are also intact.
Since the fiber optic cable member is incorporated into the lifting core of the roundsling disclosed in U.S. Pat. No. 5,651,572, it tends to develop somewhat similar breaking or snapping characteristics as the lifting core fiber materials. If the fibers of lifting core yarn break or fracture, then the fiber optic cable will also be damaged which will prevent the transmission of light from one end to the other end of the emerging fiber optic cable. If the light fails to pass from one end of the signal fiber optic cable to the other end, then the rigger is warned that the lifting core strands may be damaged, and to remove the protective cover from the roundsling for further inspection. If, upon inspection, it is determined that the roundsling was damaged, it will be immediately removed from service, and replaced with a new sling.
Although the apparatus disclosed in U.S. Pat. No. 5,651,572 is currently the leading product for determining whether the lifting core yarns of a synthetic sling have snapped or been damaged, in the stages where the sling has been subjected to an overload condition, the fiber optic signal strand still does not have the identical stretching properties of the load-bearing core yarns. Accordingly, unless the fiber optic cable breaks completely, some light may still be able to traverse the entire length of the fiber optic cable such that the degradation in the intensity of the light may be imperceptible to the naked eye.
Alternatively, the fiber optic cable, being more brittle than the synthetic core material, may be damaged by normal handling (and dropping) of the sling, or at a force less than the rated capacity of the sling. In such cases, the light transmission through the fiber optic cable may be disrupted causing the fiber optic cable to indicate an overload condition when, in fact, no overload condition was reached.
Finally, under other excessive or damage-causing situations (e.g., excessive heat, acidic or chemical exposure, and ultraviolet exposure) it can be expected that the fiber optic cable will be affected differently than the synthetic strands of the lifting core. If, for example, a sling with the fiber optic signal cable is exposed to certain chemicals, the fiber optic signal cable may be relatively unaffected (or only its exterior surface is affected leaving the light path through the center of the cable unscathed), while the lifting core has been degraded to the point where it no longer meets its load rating. Therefore, as stated previously, the need to precisely determine whether the load bearing core of a synthetic sling was subjected to an excessive or damage-causing situation still exists.